The present invention relates to traction modifiers, and more particularly, to positive drives including secondary clutch means.
Traction modifing devices have become popular for use in vehicles, in the drive train between the vehicle transmission and the driven wheels. The primary function of a traction modifier is to facilitate driving on slippery surfaces and off-road conditions. These devices are especially useful where one of the driven wheels momentarily has less traction than the other driven wheel.
Traction modifying devices are generally categorized in three classes:
1. Limited slip differentials such as illustrated in U.S Pat. Nos. 3,611,833 (nonbiased), and 3,614,717 (spring biased); PA1 2. Locking differentials such as illustrated in U.S. Pat. Nos. 1,111,728 (manually actuated), 2,978,929 (hydraulically operated), and 3,831,462 (speed responsive); and PA1 3. Positive drives such as illustrated in U.S. Pat. Nos. 1,477,311; 2,060,558; and 2,179,923. PA1 (1) The input drives the inside (slower turning) wheel while the outside wheel turns freely; and PA1 (2) The input retards the rotation of the outside (faster turning) wheel while the clutch driving the inside wheel becomes disengaged. Thus, the input of the positive drive oscillates between a driving condition and a braking condition and causes a characteristic referred to as "shudder." This condition will be described in greater detail in connection with the description of the preferred embodiments.
In general, a positive drive includes an input member adapted to be driven by the input driving torque transmitted from the power source and first and second output members adapted to drive the vehicle wheels. First and second clutch means are operably associated with the first and second output members, respectively, and each of the clutch means has a disengaged mode and an engaged mode. In the disengaged mode of each clutch means, its respective output member rotates relative to the input member, while in the engaged mode, the output member is maintained in a predetermined rotational relationship with the input member. First and second cam means are operably associated with the first and second clutch means, respectively to move the respective clutch means from the disengaged mode to the engaged mode in response to movement of the cam means from a neutral position to an actuated position.
During straight driving movement of a vehicle employing a positive drive, engine power is transmitted approximately equally to the driven wheels, which rotate at the same speed. During a potential spin-out condition (e.g., one wheel on ice and the other on pavement), engine power is transmitted to the driven wheels in proportion to their instantaneous traction limitations, whereby the wheels are still driven at the same speed.
When the vehicle turns a tight corner, power is transmitted only to the slower moving wheel, while the faster moving wheel is permitted to free-wheel, relative to the input. When the vehicle is making a gradual turn, the positive drive transmits slightly more torque to the inside wheel than to the outside wheel. Thus, a positive drive performs in a manner similar to an open differential during operating conditions which would make an open differential desirable, and performs in a manner similar to a rigid axle when operating conditions would make a rigid axle desirable.
An important distinction between positive drives and locking differentials is that in a locking differential, the side gears are interconnected by the pinions, and when the single clutch is actuated to lock one side gear to the case, both side gears are effectively locked to the case. In addition, lockup of a locking differential normally occurs only in response to a predetermined speed differential between the two outputs (side gears). In contrast, a positive drive includes two clutches, either of which may be engaged independently of the other. In addition, engagement of the clutches is the means of torque transmission in the normal drive mode, and when there is a speed differential between the two outputs, only the clutch transmitting torque to the slower turning wheel is engaged.
Although the independent actuation of the clutches of a positive drive provides better operating characteristics than other forms of traction modifiers, under most operating conditions, one operating condition has been identified in which the independent actuation of the clutches may result in an undesirable performance characteristic.
It has been discovered that when a vehicle utilizing a manual clutch transmission is in a tight turn, in low gear, and the clutch is suddenly disengaged, it is possible for the positive drive unit to oscillate between two different conditions: